The well-known DIMM (Dual In-line Memory Module) board has been used for years, in various forms, to provide memory expansion. A typical DIMM includes a conventional PCB (printed circuit board) with memory devices and supporting digital logic devices mounted on both sides. The DIMM is typically mounted in the host computer system by inserting a contact-bearing edge of the DIMM into a card edge connector. Systems that employ DIMMs provide limited space for such devices and conventional DIMM-based solutions have typically provided only a moderate amount of memory expansion.
As die sizes increase, the limited surface area available on conventional DIMMs limits the number of devices that may be carried on a memory expansion module devised according to conventional DIMM techniques. Further, as bus speeds have increased, fewer devices per channel can be reliably addressed with a DIMM-based solution. For example, 288 ICs or devices per channel may be addressed using the SDRAM-100 bus protocol with an unbuffered DIMM. Using the DDR-200 bus protocol, approximately 144 devices may be addressed per channel. With the DDR2-400 bus protocol, only 72 devices per channel may be addressed. This constraint has led to the development of the fully-buffered DIMM (FB-DIMM) with buffered C/A and data in which 288 devices per channel may be addressed. With the FB-DIMM, not only has capacity increased, pin count has declined to approximately 69 from the approximately 240 pins previously required.
The FB-DIMM circuit solution is expected to offer practical motherboard memory capacities of up to about 192 gigabytes with six channels and eight DIMMs per channel and two ranks per DIMM using one gigabyte DRAMs. This solution should also be adaptable to next generation technologies and should exhibit significant downward compatibility.
This great improvement has, however, come with some cost and will eventually be self-limiting. The basic principle of systems that employ FB-DIMM relies upon a point-to-point or serial addressing scheme rather than the parallel multi-drop interface that dictates non-buffered DIMM addressing. That is, one DIMM is in point-to-point relationship with the memory controller and each DIMM is in point-to-point relationship with adjacent DIMMs. Consequently, as bus speeds increase, the number of DIMMs on a bus will decline as the discontinuities caused by the chain of point-to-point connections from the controller to the “last” DIMM become magnified in effect as speeds increase. Consequently, methods to increase the capacity of a single DIMM find value in contemporary memory and computing systems.
There are several known methods to improve the limited capacity of a DIMM or other circuit board. In one strategy, for example, small circuit boards (daughter cards) are connected to the DIMM to provide extra mounting space. The additional connection may cause, however, flawed signal integrity for the data signals passing from the DIMM to the daughter card while the additional thickness of the daughter card(s) increases the profile of the DIMM.
Multiple die packages (MDP) are also used to increase DIMM capacity while preserving profile conformity. This scheme increases the capacity of the memory devices on the DIMM by including multiple semiconductor die in a single device package. The additional heat generated by the multiple die typically requires, however, additional cooling capabilities to operate at maximum operating speed. Further, the MDP scheme may exhibit increased costs because of increased yield loss from packaging together multiple die that are not fully pre-tested.
Stacked packages or “stacks” are yet another strategy used to increase circuit board capacity. This scheme increases capacity by stacking packaged integrated circuits to create a stacked high-density circuit module for mounting on the circuit board. In some techniques, flexible conductors are used to selectively interconnect packaged integrated circuits in such stacks.
Staktek Group LP has developed multiple innovations in memory module design and applications including stacks and larger modules. Some designs aggregate several packaged ICs on plug-in modules that replace conventional DIMMs (including, for example, fully buffered, registered or simple DIMM designs).
As signal management is brought on-board and capacities and consequent thermal issues multiply, circuits other than memory are increasingly included in memory modules. The use of other circuitry that may exhibit a profile or dimensionality that differs from that of the memory circuits can increase manufacturing complexity. Consequently, what is needed are methods and systems to adapt flex circuit-based memory modules to more readily incorporate integrated circuit packages of a variety of sizes and dimensions.